A drop-in replacement for argparse that allows options to also be set
via config files and/or environment variables.

Applications with more than a handful of user-settable options are best
configured through a combination of command line args, config files,
hard-coded defaults, and in some cases, environment variables.

Python's command line parsing modules such as argparse have very
limited support for config files and environment variables, so this
module extends argparse to add these features.
